Wet strength resins are often added to paper and paperboard at the time of manufacture. In the absence of wet strength resins, paper normally retains only 3% to 5% of its strength after being wetted with water. However, paper made with wet strength resin generally retains at least 10%-50% of its strength when wet. Wet strength is useful in a wide variety of paper applications, some examples of which are towelling, milk and juice cartons, paper bags, and liner board for corrugated containers.
Dry strength is also a critical paper property, particularly in light of the recent trend for paper manufacturers to use high yield wood pulps in paper in order to achieve lower costs. These high yield wood pulps generally yield paper with significantly reduced strength when compared to paper made from highly refined pulps.
Commercially available wet strength resins include Kymene.RTM.557H, Kymene.RTM.557LX, Kymene.RTM.557SLX, Kymene.RTM.557ULX, Kymene.RTM.Plus, Kymene.RTM.450 and Kymene.RTM.736 wet strength resins, available from Hercules Incorporated, Wilmington, Del. Wet strength resins, such as those listed above, also provide increased dry strength to paper.
Because of increased commercial emphasis on developing paper products based on recovered cellulose, there is growing interest in developing paper which is readily repulpable. Paper and paperboard waste materials are difficult to repulp in aqueous systems without special chemical treatment when they contain wet strength resins. Previous to the instant invention, improving the repulpability of paper containing wet strength resins has been achieved by modifying the repulping conditions by incorporation of repulping agents, usually inorganic oxidizing agents or chlorine containing materials. Such methods are disclosed, for example, in U.S. Pat. No. 2,872,313 to House et al., U.S. Pat. No. 3,427,217 to Miller, and U.S. Pat. No. 5,330,619 to Henry et al., by Schmalz, in TAPPI, 44, no. 4, pp 275-280, April, 1961, by Espy in European Patent Application Publication No. 585,955-A, and by Caropreso et al., in PCT International Publication No. WO94/20682. Since many of the repulping processes used for paper containing wet and/or dry strength resins result in formation of environmentally undesirable chlorine-containing degradation products, involve strong oxidizing agents, or proceed slowly, there is need for improved resins that are capable of providing wet and dry strength paper that is readily repulpable without added repulping agents or special chemical treatments.
Resins similar to those used for imparting strength to paper are also often used as creping adhesives. In the manufacture of some paper products such as facial tissue, bathroom tissue, or paper towers, the paper web is conventionally subjected to a creping process in order to give it desirable textural characteristics, such as softness and bulk. The creping process typically involves adhering the web, a cellulose web in the case of paper, to a rotating creping cylinder, such as the apparatus known as a Yankee dryer, and then dislodging the adhered web with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker.
The severity of this creping action is dependent upon a number of factors, including the degree of adhesion between the web and the surface of the creping cylinder. Greater adhesion causes increase softness, although generally with some loss of strength. In order to increase adhesion, a creping adhesive may be used to enhance any naturally occurring adhesion that the web may have due to its water content, which will vary widely depending on the extent to which the web has been previously dried. Creping adhesives should also prevent wear of the dryer surface and provide lubrication between the doctor blade and the dryer surface and reduce chemical corrosion, as well as controlling the extent of creping. A creping adhesive coating that adheres the sheet just tightly enough to the drum will give a good crepe, imparting absorbance and softness with the least possible loss of paper strength. If adhesion to the dryer drum is too strong, the sheet may pick or even "plug", i.e., underride the doctor blade, and wrap around the dryer drum. If there is not enough adhesion, the sheet will lift off too easily and undergo too little creping.
The creping adhesive, usually as an aqueous solution or dispersion, is generally sprayed onto the surface of the creping cylinder or drum, e.g., a Yankee dryer. This improves heat transfer, allowing more efficient drying of the sheet. If the pulp furnish sticks too strongly to the creping cylinder, release agents can be sprayed on the cylinder. The release agents are typically hydrocarbon oils. These agents aid in the uniform release of the tissue web at the creping blade, and also lubricate and protect the blade from excessive wear.
A creping adhesive composition is disclosed in U.S. Pat. No. 5,187,219 to Furman. The composition comprises a water-soluble glyoxylated acrylamide/diallyldimethylammonium chloride polymer and a water-soluble polyol having a molecular weight below 3000 as a plasticizer for the polymer.
U.S. Pat. No. 5,246,544 to Hollenberg et al., discloses a reversibly crosslinked creping adhesive which contains a nonself-crosslinkable material that is a polymer or oligomer having functional groups that can be crosslinked by ionic crosslinking and at least one metal, cationic crosslinking agent having a valence of four or more. The adhesive can also contain additives to modify the mechanical properties of the crosslinked polymers, e.g., glycols, polyethylene glycols, and other polyols such as simple sugars and oligosaccharides.
Polyaminoamide/epichlorohydrin creping adhesives are disclosed in U.S. Pat. No. 5,338,807 to Espy et al. and in Canadian Patent 979,579 Giles et al.
U.S. Pat. No. 5,374,334 to Sommese et al., discloses a creping adhesive which is a crosslinked vinyl amine/vinyl alcohol polymer containing from about 1 to about 99% vinyl amine. Epichlorohydrin is disclosed as a crosslinking agent.
U.S. Pat. No. 4,684,439 and 4,788,243 to Soerens, disclose creping adhesives comprising mixtures of polyvinyl alcohol and water soluble thermoplastic polyamide resin comprising the reaction product of a polyalkylenepolyamine, a saturated aliphatic dibasic carboxylic acid and a poly(oxyethylene) diamine.
In U.S. Pat. Nos. 4,501,640 and 4,528,316 to Soerens, there is disclosed a creping adhesive comprising a mixture of polyvinyl alcohol and a water soluble, thermosetting cationic polyamide resin.
Commercially available creping adhesives include Crepetrol.RTM.190, Crepetrol.RTM.290, Crepetrol.RTM.80E cationic polymers, available from Hercules Incorporated, Wilmington, Del.
There is a need for improved creping adhesives with adhesion improved over that obtained with the commonly used polyaminoamide/epichlorohydrin creping adhesives.